1. Field of the Invention
The present invention relates to a liquid crystal device which can be used, for example, in a portable information terminal having a pen based input, OA equipment such as a television, or a display having a wide viewing angle, and to a method for producing the same. More particularly, the present invention relates to a liquid crystal display device including a liquid crystal region surrounded by a polymer wall which operates a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, a ferroelectric liquid crystal (FLC) display mode, a light scattering mode or the like; and a method for producing the same.
2. Description of the Related Art
Nowadays, various kinds of display modes have been employed for a liquid crystal display (LCD) device. For example, LCDs for providing a display in a twisted nematic (TN) mode or a super twisted nematic (STN) mode using nematic liquid crystal molecules are practically used as LCDs utilizing electrooptic characteristics for performing a display. In addition, LCDs utilizing ferroelectric liquid crystal material can be put into practical use.
Moreover, a mode for electrically controlling a transparent state and a scattering state of a liquid crystal material by the use of birefringence of the liquid crystal material has recently been proposed in, for example, Japanese Laid-Open National Patent Publication No. 58-501631. In an LCD operating in accordance with this mode, a display medium in which liquid crystal droplets are dispersed in a polymer is interposed between a pair of substrates opposed to each other. This type of LCD is called a polymer dispersed liquid crystal (PDLC) display device. In a PDLC display device, a display is performed in the following manner. When a voltage is applied to the liquid crystal layer, the orientations of the liquid crystal molecules are aligned towards the direction of the electric field. As a result, the ordinary refractive index of the liquid crystal molecules is matched with the refractive index of the polymer serving as a support medium, whereby a transparent state is obtained. On the other hand, when no voltage is applied to the liquid crystal layer, the random orientations of the liquid crystal molecules cause light scattering, whereby an opaque state is obtained. By controlling orientations of liquid crystal molecules between the transparent state and the opaque state, display can be performed.
According to a method for producing the above-mentioned PDLC display device, the liquid crystal droplets are formed by utilizing the phase-separation between the polymer and the liquid crystal material. Thus, the shapes and size of the liquid crystal droplets are not uniform, and it is difficult to precisely control arrangements of the liquid crystal droplets in a direction along the surface of the substrates. Consequently, different driving voltages are required to be applied to the respective liquid crystal droplets to degrade the steepness of the threshold value exhibiting the electrooptic characteristics, resulting in a relatively high driving voltage.
In order to solve the problems of the above-mentioned conventional PDLC display device, Japanese Laid-Open Patent Publication No. 6-301015 assigned to the same assignee of the present application discloses a new display mode using an improved PDLC display device. In preparation of this LCD, ultraviolet rays are radiated onto the mixture of a liquid crystal material and a photopolymerizable compound through a photomask and the like so that some regions are irradiated with relatively intense light and other regions are irradiated with relatively weak light. As a result, the polymer is aggregated in the regions irradiated with the relatively intense light to form polymer walls, while the liquid crystal material is aggregated in the regions irradiated with the relatively weak light to form a liquid crystal region. According to this method, liquid crystal droplets can be uniformly formed in shape and size. Also, it is possible to precisely control the arrangement of liquid crystal droplets, i.e., pixels, in a direction along a surface of the substrate.
In the above-mentioned liquid crystal display device, the substrate includes: a display section in which transparent electrodes made of a material for reducing the amount of UV rays to be transmitted such as ITO (Indium Tin Oxide) is formed; and a terminal section (a non-display section) formed around the display section. Polymer walls are formed between the adjacent transparent electrodes, and between transparent electrodes of two opposed substrates and the terminal region. An overlapped region of the electrodes formed on the substrates opposed to each other serves as a pixel. Typically, a distance between the pixel in the periphery of the display section and the end portion of the substrate is larger than that between pixels in the display section. Therefore, in the boundary region between the display section and the non-display section, there are necessarily some large UV ray irradiated areas and other smaller UV irradiated areas. For example, in the corner of the display section, a distance between the pixel present at the corner and the pixel adjacent thereto is smaller than a distance between these pixels and the end portion of the substrate. Therefore, the area irradiated with UV rays in the latter case is larger than that in the former case.
Generally, the polymerization of the photopolymerizable compound occurs more easily in the large area irradiated with UV rays than in the small area. Thus, the polymerization more rapidly proceed in the region between the end portion of the substrate than in the region between pixels in the boundary region between the display section and the non-display section. With the polymerization of photopolymerizable compounds, since a large amount of the photopolymerizable compound is converted to a polymer in the region between the pixel and the end portion of the substrate, photopolymerizable compounds migrate from the region between pixels to the region between the pixel and the end portion of the substrate, decreasing a concentration of the polymerizable compound in the region between pixels. Furthermore, some polymers produced in the region between pixels also migrate from the region between pixels to the region between the pixel and the end portion of the substrate, resulting in poor formation of polymer walls in the region between pixels.
As a result, a ratio of polymer walls and liquid crystal regions in the peripheral region of the display section differs from that in the central region of the display section. The amount of light transmitted through the liquid crystal region and the polymer wall, which have different refractive indices, in the peripheral region of the display section differs from that in the central region of the display section. In this way, since the amount of transmitted light in the peripheral region of the display section differs from that in the central region of the display section, the tone obtained in the peripheral region is different from that obtained in the central region. Thus, a conventional liquid crystal display device is disadvantageous in that it is difficult to obtain uniform display over the entire display region.
Japanese Laid-Open Patent Publication Nos. 61-215524 and 63-137213 disclose a method for increasing the reliability of a panel by adhering a pair of substrates using a two-layered structure sealing material or for enhancing the panel strength so as to reduce unevenness of a gap between the substrates. However, the above-mentioned sealing material having the two-layered structure has not been used for the purpose of enhancing display quality of a liquid crystal display device by preventing migration of photopolymerizable compounds and polymers from the region between pixels to the region between the pixel and the end portion of the substrate.